Acid-induced topological morphology modulation of graphitic carbon nitride homojunctions as advanced metal-free catalysts for OER and pollutant degradation

•A controllable acid-induced molecular self-assembly route is first proposed to prepare hierarchical CN homojunction.•The topological morphology of CN homojunction can be easily adjusted.•The hierarchical CN homojunction endows a synergistic effect of active sites, charge transfer and separation.•The optimized CN homojunction shows superior OER activity and high ability for pollutant degradation.